Method and apparatus for reducing non-normal incidence distortion in glazing films

ABSTRACT

A method of manufacturing a polymer film includes melting a resin, extruding the melted resin through a die to produce a polymer film, shaping the polymer film, cooling the polymer film, illuminating a surface with a light source, capturing an image of a shadow cast by the polymer film on the surface while the polymer film is between the light source and the surface, calculating a frequency spectrum of at least a portion of the captured image, and adjusting a process parameter of the melting, the extruding, the shaping, or the cooling based on the frequency spectrum.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 17/103,397, filed Nov. 24, 2020, which claims the benefit of U.S. Provisional Application No. 62/942,943, filed Dec. 3, 2019, the contents of each of which are expressly incorporated herein by reference.

STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT

Not Applicable

BACKGROUND 1. Technical Field

The present disclosure relates generally to transparent coverings for windows, eyewear, or display screens and, more particularly, transparent coverings for use in non-normal incidence applications such as vehicle windshields.

2. Related Art

In various contexts, it is advantageous to affix transparent coverings to a substrate. Windows of buildings or vehicles may be covered with transparent window films for tinting (e.g. for privacy), for thermal insulation, to block ultraviolet (UV) radiation, or for decoration. Protective eyewear (e.g. goggles, glasses, and facemasks for off-road vehicle use, medical procedures, etc.) may be covered with a stack of transparent lenses for easy tear-away as the eyewear becomes dirty and obstructs the wearer's vision. Transparent lenses may similarly be applied to a telescopic sight or scope as may be used with a rifle, for example. Display screens of mobile phones, personal computers, ATMs and vending terminals, etc. may be covered with protective lenses to prevent damage to the underlying screen or block side viewing (e.g. for privacy and security in public places). While the majority of such applications transmit light to an observer at normal incidence, non-normal incidence applications exist as well. In the case of automobile windshields, for example, there has been a trend to increase the angle of incidence to 60-70 degrees from normal or even higher in an effort to reduce drag and improve fuel efficiency. The same is true of windshields for various other vehicles including trucks, planes, buses, trains, etc.

BRIEF SUMMARY

The co-inventors have discovered an increase in optical distortion when transparent coverings (e.g. glazing films) are applied at high angles of incidence (e.g. greater than 60 degrees from normal) as in the case of transparent coverings applied to vehicle windshields. The present disclosure contemplates various apparatuses and methods for manufacturing polymer films that overcome this difficulty, as well as polymer films made in accordance therewith.

One aspect of the embodiments of the disclosure is a method of manufacturing a polymer film. The method may include melting a resin, extruding the melted resin through a die to produce a polymer film, shaping the polymer film, cooling the polymer film, capturing an image of a test pattern through the polymer film, calculating a modulation transfer function value from the image, and adjusting a process parameter of the melting, the extruding, the shaping, or the cooling based on the calculated modulation transfer function value.

The process parameter may be a temperature setting of a heater used in the melting.

The process parameter may be a rotation speed of an extrusion screw used in the extruding.

The process parameter may be a rotation speed of a roller used in the shaping or the cooling.

The method may include capturing an additional image of the test pattern through the polymer film with the polymer film at a different angle relative to the test pattern and calculating an additional modulation transfer function value from the additional image. The adjusting may be based on the calculated additional modulation transfer function value. During the capturing of the image, the polymer film may be at an angle relative to the test pattern of 55-65 degrees. During the capturing of the additional image of the test pattern through the polymer film, the polymer film may be at an angle relative to the test pattern of 65-75 degrees.

The capturing of the image may be performed by an imaging radiometer 10-30 meters from the test pattern. The capturing of the image may be performed with the test pattern 1-10 meters from the polymer film.

The test pattern may comprise line pairs.

The method may include capturing a baseline image of the test pattern that is not taken through the polymer film and calculating a baseline modulation transfer function value from the baseline image. The adjusting may be based on a difference between the calculated modulation transfer function value and the calculated baseline modulation transfer function value. During the capturing of the image, the polymer film may be at an angle relative to the test pattern of 55-65 degrees. The adjusting may be performed such that the difference between the calculated modulation transfer function value and the calculated baseline modulation transfer function value is kept below 0.12. The method may include capturing an additional image of the test pattern through the polymer film with the polymer film at an angle relative to the test pattern of 65-75 degrees and calculating an additional modulation transfer function value from the additional image. The adjusting may be performed such that the difference between the calculated additional modulation transfer function and the calculated baseline modulation transfer function is kept below 0.38. During the capturing of the image, the polymer film may be at an angle relative to the test pattern of 60 degrees. During the capturing of the additional image, the polymer film may be at an angle relative to the test pattern of 70 degrees.

The polymer film may be a biaxially-oriented polyethylene terephthalate film.

The process parameter may affect a density variation of the polymer film.

The process parameter may affect a refractive index variation in the polymer film. The process parameter may affect a frequency of refractive index changes on the order of 0.010 in the polymer film.

Another aspect of the embodiments of the disclosure is a polymer film. The polymer film may have a density variation such that a difference between i) a first modulation transfer function value calculated from an image of a test pattern captured through the polymer film with the polymer film at an angle of 60 degrees relative to the test pattern and ii) a baseline modulation transfer function calculated from an image of the test pattern that is not taken through the polymer film is less than 0.12. The density variation may be such that a difference between i) a second modulation transfer function value calculated from an image of a test pattern captured through the polymer film with the polymer film at an angle of 70 degrees relative to the test pattern and ii) the baseline modulation transfer function is less than 0.38.

Another aspect of the embodiments of the disclosure is an apparatus for manufacturing a polymer film. The apparatus may include an extruder assembly for melting a resin and extruding the melted resin through a die to produce a polymer film, a roller for shaping and/or cooling the polymer film, an image sensor for capturing an image of a test pattern through the polymer film, and a computer for calculating a modulation transfer function value from the image and adjusting a process parameter of the extruder assembly or the roller based on the calculated modulation transfer function value.

Another aspect of the embodiments of the disclosure is a method of manufacturing a polymer film. The method may include melting a resin, extruding the melted resin through a die to produce a polymer film, shaping the polymer film, cooling the polymer film, illuminating a surface with a light source, capturing an image of a shadow cast by the polymer film on the surface while the polymer film is between the light source and the surface, calculating a frequency spectrum of at least a portion of the captured image, and adjusting a process parameter of the melting, the extruding, the shaping, or the cooling based on the frequency spectrum.

The process parameter may be a temperature setting of a heater used in the melting.

The process parameter may be a rotation speed of an extrusion screw used in the extruding.

The process parameter may be a rotation speed of a roller used in the shaping or the cooling.

The polymer film may be a multi-layer film. The process parameter may be a parameter for applying an adhesive used to stack two or more layers of the multi-layer film.

The calculating may include calculating the frequency spectrum of a cross-section of the shadow image corresponding to a slice of the polymer film in a transverse direction of the polymer film.

During the capturing of the image, the polymer film may be at an angle of incidence greater than 10 degrees with respect to the light source. During the capturing of the image, the polymer film may be at an angle of incidence of 50-70 degrees with respect to the light source. During the capturing of the image, the polymer film may be at an angle of incidence of 55-65 degrees with respect to the light source. The method may include capturing an additional image of a shadow cast by the polymer film on the surface while the polymer film is between the light source and the surface at an angle of incidence less than 10 degrees with respect to the light source and calculating an additional frequency spectrum of at least a portion of the captured additional image. The adjusting may be further based on the additional frequency spectrum. During the capturing of the additional image, the polymer film may be at an angle of incidence less than 5 degrees with respect to the light source.

The adjusting may be performed such that the frequency spectrum is kept below a first threshold within a first predefined frequency range. The adjusting may be performed such that the frequency spectrum is further kept below a second threshold within a second predefined frequency range.

The adjusting may be performed such that a maximum of the frequency spectrum is kept outside of a predefined frequency range. The predefined frequency range may be 0.5-2 cycles per millimeter.

The polymer film may be a biaxially-oriented polyethylene terephthalate (BOPET) film, a thermoplastic polyurethane (TPU) film, or a biaxially-oriented polypropylene (BOPP) film. The polymer film 200 may have a visible light transmission (VLT) greater than 89%.

The process parameter may affect a density variation of the polymer film.

The process parameter may affect a refractive index variation in the polymer film. The process parameter may affect a frequency of refractive index changes on the order of 0.010 in the polymer film.

Another aspect of the embodiments of the disclosure is a polymer film. The polymer film may have a density variation such that a maximum value of a frequency spectrum calculated from grayscale image data of a transverse cross-section of a shadow cast by the polymer film at 60 degrees angle of incidence with respect to a light source is at a spatial frequency greater than 2 cycles per millimeter.

Another aspect of the embodiments of the present disclosure is an apparatus for manufacturing a polymer film. The apparatus may include an extruder assembly for melting a resin and extruding the melted resin through a die to produce a polymer film, a roller for shaping and/or cooling the polymer film, a light source for illuminating a surface, an image sensor for capturing an image of a shadow cast by the polymer film on the surface while the polymer film is between the light source and the surface, and a computer for calculating a frequency spectrum of at least a portion of the captured image and adjusting a process parameter of the extruder assembly or the roller based on the calculated frequency spectrum.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the various embodiments disclosed herein will be better understood with respect to the following description and drawings, in which like numbers refer to like parts throughout, and in which:

FIG. 1 shows an example apparatus for manufacturing a polymer film according to an embodiment of the present disclosure;

FIG. 2 shows an example graphical representation of modulation transfer function (MTF) data for various brands of biaxially-oriented polyethelene terephthalate (BoPET) at various angles of incidence.

FIG. 3 shows another example graphical representation of MTF data for various brands of BoPET at various angles of incidence.

FIG. 4 shows an example operational flow according to an embodiment of the present disclosure;

FIG. 5 shows an example operational flow of step 440 of FIG. 4;

FIG. 6 shows another example apparatus for manufacturing a polymer film according to an embodiment of the present disclosure;

FIG. 7A shows an example image of a shadow cast by bare glass on a surface;

FIG. 7B shows an example graphical representation of grayscale data of the image of FIG. 7A;

FIG. 7C shows an example graphical representation of a frequency spectrum of the grayscale data of FIG. 7B;

FIG. 8A shows an example image of a shadow cast by a polymer film on a surface;

FIG. 8B shows an example graphical representation of grayscale data of the image of FIG. 8A;

FIG. 8C shows an example graphical representation of a frequency spectrum of the grayscale data of FIG. 8B;

FIG. 9A shows another example image of a shadow cast by a polymer film on a surface;

FIG. 9B shows an example graphical representation of grayscale data of the image of FIG. 9A;

FIG. 9C shows an example graphical representation of a frequency spectrum of the grayscale data of FIG. 9B;

FIG. 10A shows another example image of a shadow cast by a polymer film on a surface;

FIG. 10B shows an example graphical representation of grayscale data of the image of FIG. 10A;

FIG. 10C shows an example graphical representation of a frequency spectrum of the grayscale data of FIG. 10B;

FIG. 11A shows another example image of a shadow cast by a polymer film on a surface;

FIG. 11B shows an example graphical representation of grayscale data of the image of FIG. 11A;

FIG. 11C shows an example graphical representation of a frequency spectrum of the grayscale data of FIG. 11B;

FIG. 12A shows another example image of a shadow cast by a polymer film on a surface;

FIG. 12B shows an example graphical representation of grayscale data of the image of FIG. 12A;

FIG. 12C shows another example graphical representation of the grayscale data of the image of FIG. 12A;

FIG. 12D shows the same example image of FIG. 12A but with an area indicated corresponding to the grayscale data of FIG. 12C;

FIG. 12E shows an example graphical representation of a frequency spectrum of the grayscale data of FIG. 12C;

FIG. 12F shows another example graphical representation of a frequency spectrum of the grayscale data of FIG. 12C;

FIG. 12G shows another example graphical representation of a frequency spectrum of the grayscale data of FIG. 12C;

FIG. 13A shows another example image of a shadow cast by a polymer film on a surface;

FIG. 13B shows an example graphical representation of grayscale data of the image of FIG. 13A;

FIG. 13C shows an example graphical representation of a frequency spectrum of the grayscale data of FIG. 13B; and

FIG. 14 shows another example operational flow according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

The present disclosure encompasses various polymer films and polymer film manufacturing apparatuses and methods. The detailed description set forth below in connection with the appended drawings is intended as a description of several currently contemplated embodiments. It is not intended to represent the only form in which the disclosed subject matter may be developed or utilized. The description sets forth the functions and features in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions may be accomplished by different embodiments that are also intended to be encompassed within the scope of the present disclosure. It is further understood that the use of relational terms such as first and second and the like are used solely to distinguish one from another entity without necessarily requiring or implying any actual such relationship or order between such entities.

FIG. 1 shows an example apparatus 100 for manufacturing a polymer film 200 such as a biaxially oriented polyethelene terephthalate (BoPET) film according to an embodiment of the present disclosure. During or after the manufacture of the polymer film 200, an image sensor 110, such as an imaging radiometer or camera, captures an image of a test pattern 300 through the polymer film 200. A computer 120 calculates a modulation transfer function (MTF) value from the captured image and feeds the result back into the manufacturing process of the polymer film 200. In this way, the apparatus 100 may adjust one or more process parameters that have been found by the inventor to influence optical distortion in the manufactured polymer film 200.

FIGS. 2 and 3 show example graphical representations of MTF data for various brands of BoPET films at various angles of incidence. The data, shown as a line graph in FIG. 2 and a bar graph in FIG. 3, represents the difference in measured MTF values (ranging from 0.0000 to 0.8000 on the y-axis) between an image of a test pattern 300 viewed through a transparent sample and an image of the test pattern 300 directly, as a function of angle of incidence (ranging from 0 to 80 degrees from normal) for four BoPET film samples T10, T9, T8, and T7, as well as for a 9.75 Med. 3-Ply (i.e., medical grade) BoPET film sample and a bare glass sample. As can be seen, for all of the polymer film samples, the distortion represented by the MTF difference gradually increases with angle of incidence until around 60 degrees, at which point the data exhibits a “knee” indicative of a sudden increase in distortion as the MTF of the films starts to collapse. This sudden worsening of the MTF of polymer films at around 60 degrees, which is not present in bare glass, is believed to be due to the occurrence of random small changes in refractive index on the order of 0.010 caused by density variations across the polymer film as it is extruded and cooled during manufacturing. By feeding MTF data back into the manufacturing process, it is thus possible to tune the relevant process parameters to produce a polymer film having improved distortion characteristics at off-normal incidence.

Referring back to FIG. 1, an example extruder assembly 130 of the apparatus 100 may include a hopper 131 for loading polymer resin (e.g. pellets, beads, etc. which may include various materials to be compounded), a heater 133 for providing heat to the extruder assembly 130 to melt the resin, an extrusion screw 135 for moving the resin forward through one or more heated regions of the extruder assembly 130 (e.g. by rotating within a barrel), and a die 137 having a desired shape through which the melted resin is forced to produce the resulting polymer film 200. The polymer film 200 may thereafter be cooled and/or further shaped by one or more downstream rollers 140, eventually bringing the polymer film 200 to its final thickness and shape.

As noted above, it is believed that density variations across the polymer film as it is extruded and cooled cause changes in the index of refraction that result in the increased distortion found at higher angles of incidence. Therefore, it is contemplated that the apparatus 100 may be configured to adjust one or more process parameters that affect the density variation of the polymer film 200 and/or the refractive index variation in the polymer film 200. Relevant process parameters may include, for example, a temperature setting of the heater 133 used in melting the resin (e.g. absolute temperature or relative temperatures of a gradient or profile of a plurality of heated regions of the extruder assembly 130), a rotation speed of the extrusion screw 135 (which may determine melting time as well as degree of mixing of the resin), and/or a rotation speed of the one or more rollers 140 (which may determine cooling time and/or a degree of force acting on the polymer film 200 during or prior to cooling to stretch or otherwise shape the polymer film 200 in longitudinal and/or transverse directions while the polymer film 200 is still pliable). The computer 120 may be programmed to adjust one or more such process parameters or any other relevant process parameters of the melting, extruding, shaping, or cooling based on a calculated MTF value associated with the manufactured polymer film 200. In this way, the distortion of the polymer film 200 may be optimized for the intended angle of incidence at which the polymer film 200 will be used.

The MTF value calculated by the computer 120 may be, for example, a single value of a modulation transfer function corresponding to a specific spatial frequency (e.g. a contrast percentage when resolving a specific number of line pairs per millimeter), an average value of a modulation transfer function over a range of spatial frequencies, or any other value representative of or derived from a modulation transfer function. In the example of the apparatus 100 shown in FIG. 1, a test pattern 300 comprising line pairs defining one or more spatial frequencies (e.g. 5 LP/mm) is set up behind the polymer film 200, and an image sensor 110 (e.g. a 40-megapixel imaging radiometer) is positioned to capture an image of the test pattern 300 through the polymer film 200. The test pattern may be 1-10 meters (e.g. 5 meters) from the polymer film 200, and the image sensor 110 may be 10-30 meters (e.g. 15 meters) from the polymer film 200. The test pattern 300 may be produced by an LCD pattern generator, for example, in which case the one or more spatial frequencies may be sequentially generated. Alternatively, the test pattern 300 may be printed on a substrate and may include one or more spatial frequencies located in different regions of the substrate.

The MTF value calculated from the image captured through the polymer film 200 may be compared to a baseline MTF value calculated from a direct image of the test pattern 300 without the polymer film 200. For example, the baseline MTF value may be subtracted from the MTF value associated with the polymer film 200 such that a difference value of “0” represents no distortion caused by the polymer film 200 and a difference value of “1” represents total distortion (i.e. no resolution). In this way, a difference in measured MTF values between an image of the test pattern 300 viewed through the polymer film 200 and an image of the test pattern 300 directly may be obtained, such as difference data of the type shown in FIGS. 2 and 3. To this end, as shown in the example of FIG. 1, the test pattern 300 may be positioned relative to the polymer film 200 and the image sensor 110 such that the field of view of the image sensor 110 encompasses both a region R1 of the test pattern 300 that is behind the polymer film 200 and a region R2 of the test pattern 300 that is not behind the polymer film 200. With such arrangement, the image sensor 110 may capture a baseline image of the test pattern 300 that is not taken through the polymer film 200 but is otherwise taken under the same conditions as the image captured through the polymer film 200. Alternatively, the baseline image of the test pattern 300 may be captured at a different time under substantially the same conditions.

In order to obtain MTF values for different angles of the polymer film 200, multiple images may be taken with the polymer film 200 rotated relative to the image sensor 110 and/or test pattern 300. For example, during the capturing of a first image of the test pattern 300 through the polymer film 200, the polymer film may be at an angle relative of the test pattern 300 of 55-65 degrees (e.g. 60 degrees) and, during capturing of an additional image of the test pattern 300 through the polymer film 200, the polymer film 200 may be at an angle relative to the test pattern 300 of 65-75 degrees (e.g. 70 degrees). The computer 120 may then adjust the manufacturing process parameter(s) based on both the MTF value calculated from the first image and an additional MTF value calculated from the additional image, both relative to a baseline MTF value as described above. It is contemplated that a sufficiently distortion-free film for use at off-normal incidence (e.g. for vehicle windshields) may have an MTF value difference (relative to baseline) of below 0.12 at 60 degrees and an MTF value difference (relative to baseline) of below 0.38 at 70 degrees.

It is contemplated that the computer 120 may be programmed to adjust the process parameter(s) automatically without user input or in response to commands entered into a user interface of the computer 120. In this regard, the apparatus 100 may be set up to allow the image sensor 110 to capture images of the test pattern 300 through the polymer film 200 in a continuous process. For example, the various images described above may be captured during or after cooling while the polymer film 200 is on the roller(s) 140. In the case of multiple images at different angles of incidence, multiple image sensors 110 and/or test patterns 300 may be set up at different stages or a single image sensor 110 and/or test pattern 300 may automatically move to multiple positions. As the computer 120 calculates MTF values from the captured images, the computer 120 may continuously adjust the relevant process parameters in order to keep the desired MTF value(s) (or difference(s) from baseline thereof) below specified values. Alternatively, the apparatus 100 may be set up to capture images and make adjustments to process parameters in a batch to batch process, either automatically or by manual operation. For example, after a polymer film 200 batch is completed (or during cooling), the relevant MTF values may be calculated and the computer 120 may make adjustments to the process parameters to improve the distortion characteristics of the next batch or to optimize the distortion characteristics of the next batch for a different purpose (e.g. to minimize distortion at a different range of angles of incidence).

FIGS. 4 and 5 show an example operational flow according to an embodiment of the present disclosure, with FIG. 5 detailing an example subprocess of step 440 of FIG. 4. Referring by way of example to the apparatus 100 shown in FIG. 1, the operational flow may begin with a step 410 of melting a resin, a step 420 of extruding the melted resin through a die to produce a polymer film 200, and steps 430 and 440 of shaping (e.g. stretching) and cooling the polymer film 200. For example, steps 410, 420, 430, and 440 may be performed by an extruder assembly 130 and downstream roller(s) 140 as described in relation to FIG. 1. During or subsequent to these steps, the operational flow may include a step 450 of capturing one or more images of a test pattern 300 (e.g. using an image sensor 110). As shown in FIG. 5, the capturing of one or more images of the test pattern 300 may include a step 452 of capturing a first image of the test pattern 300 through the polymer film 200 at a first angle (e.g. around 60 degrees from normal), a step 454 of capturing an additional image of the test pattern 300 through the polymer film 200 at a second angle (e.g. around 70 degrees from normal), and a step 456 of capturing a baseline image of the test pattern 300 that is not taken through the polymer film 200. For example, the baseline image may be taken by the image sensor 110 of a region R2 of the test pattern 300 that is not behind the polymer film 200. In some cases, the baseline image 110 may be taken at a different time altogether.

Referring back to FIG. 4, following the capturing of one or more images of the test pattern 300 during or after steps 410, 420, 430, and 440, the operational flow may continue with a step 460 of calculating one or more MTF values from the captured image(s) and a step 470 of adjusting process parameter(s) based on the calculated MTF value(s). In this way, the results of calculating the one or more MTF values associated with the polymer film 200 may be fed back into the manufacturing process to tune the optical distortion properties of the polymer film 200 being produced. The calculation of MTF value(s) and/or adjusting of process parameter(s) may be performed by an appropriately programmed computer 120 as described above. In this regard, the computer 120 may include a processor or programmable circuit (e.g. FPGA, PLA, etc.) for executing program instructions (e.g. software instructions, state information, etc.). The calculating step 460 and/or adjusting step 470 may be embodied in such program instructions and stored on a non-transitory program storage medium to be executed by the computer 120.

FIG. 6 shows another example apparatus 600 for manufacturing a polymer film 200 such as a biaxially oriented polyethelene terephthalate (BoPET) film, a thermoplastic polyurethane (TPU) film, or a biaxially-oriented polypropylene (BOPP) film according to an embodiment of the present disclosure. The polymer film 200 may have a visible light transmission (VLT) greater than 89%, for example. Like the apparatus 100, the apparatus 600 may adjust one or more process parameters that have been found by the inventor to influence optical distortion in the manufactured polymer film 200. However, instead of using a test pattern 300 (see FIG. 1), the apparatus 600 may adjust the process parameter(s) based on observed distortion in a shadow cast by the polymer film 200. To this end, during or after the manufacture of the polymer film 200, the polymer film 200 may be disposed between a light source 650 such as a collimated projector and a surface 660 such as a white screen. As the surface 660 is illuminated by the light source 650, an image sensor 610 such as an imaging radiometer or camera captures an image of a shadow 670 cast by the polymer film 200 on the surface 660. A computer 620 calculates a frequency spectrum of at least a portion of the captured image 670 and feeds the result back into the manufacturing process of the polymer film 200.

The polymer film 200 may be inserted into the optical path of the light source 650 at a distance of 12-36 inches (e.g. 24 inches) from the surface 660, for example, and may be disposed at various angles of incidence relative to the light source 650. In this regard, multiple images may be taken to characterize the polymer film 200 at different angles of incidence, such as 60 degrees, which is typical of a windshield and/or 0 degrees (normal incidence). More generally, during the capturing of a first image of the shadow 670 cast by the polymer film 200, which may be used to characterize the polymer film 200 at off-normal incidence, the polymer film 200 may be at an angle of incidence greater than 10 degrees with respect to the light source 650, preferably greater than 45 degrees, more preferably 50-70 degrees or 55-65 degrees (e.g. 60 degrees). During the capturing of an additional image of the shadow 670 cast by the polymer film 200, which may be used to characterize the polymer film 200 at normal incidence, the polymer film 200 may at an angle of incidence less than 10 degrees with respect to the light source 650, preferably less than 5 degrees (e.g. 0 degrees). The computer 620 may then adjust the manufacturing process parameter(s) based on one or both of the first image and the additional image.

FIG. 7A shows an example image of a shadow 670 a cast by bare glass 200 a on the surface 660, with the bare glass 200 a having been interposed between the light source 650 and the screen 660 in place of the polymer film 200 shown in FIG. 6. In the example of FIG. 7A, the bare glass is disposed at 60 degrees angle of incidence relative to the light source 650. In particular, the bare glass is tilted toward the light source 650 (opposite to what is schematically shown in FIG. 6), such that the top portion of the shadow 670 a is cast by a portion of the bare glass that is nearer to the light source 650 and farther from the screen 660 while the bottom portion of the shadow 670 a is cast by a portion of the bare glass that is farther from the light source 650 and nearer to the screen 660. Note that the slight left-to-right “keystone” shape of the shadow 670 a is not related to the angle of incidence but rather the angle of the image sensor 610, which is positioned slightly off-axis (to the left as viewed from the light source 650). In a case where only a vertical cross-section of the shadow 670 is to be used as described below, positioning the image sensor 610 off-axis in this way does not significantly affect the analysis (and it may therefore be unnecessary to keep the image sensor 610 on-axis using a beam splitter, for example).

FIG. 7B shows an example graphical representation of grayscale data of the image of FIG. 7A. The grayscale data may be taken from a cross-section of the shadow image corresponding to a vertical slice of the bare glass. In particular, the grayscale data of FIG. 7B is the grayscale values of the 2500 pixels at the position of the vertical line 680 a in FIG. 7A. The grayscale values are shown on the y-axis of FIG. 7B for each pixel along the x-axis, with the resulting data presented as a line graph.

FIG. 7C shows an example graphical representation of a frequency spectrum of the grayscale data of FIG. 7B. The frequency spectrum may be calculated by the computer 620 using a Fast Fourier Transform (FFT) algorithm, for example. The frequency spectrum may represent the amplitude of the grayscale data at each spatial frequency, shown as cycles (peak-to-peak) per millimeter along the x-axis. Thus, to the extent that distortion observable in the shadow image exhibits periodicity along the direction of the cross-section, such distortion will show up as a spike (i.e. local maximum) at the frequency corresponding to the periodicity. As can be seen in FIG. 7C, the frequency spectrum of the bare glass does not exhibit any spikes, with the frequency spectrum staying under 0.5 up to 2 cycles/mm and staying under 0.75 above 2 cycles/mm.

FIG. 8A shows an example image of a shadow 670 b cast by a polymer film 200 b on the surface 660, with the polymer film 200 b being an example of the polymer film 200 shown in FIG. 6. In the example of FIG. 8A, the polymer film 200 b is a BOPET film sample having a hard coat formed thereon. The polymer film 200 b is oriented so that its machine direction (MD) is horizontal, just as it would be if it were mounted on a windshield. As in the case of the bare glass 200 a of FIG. 7A, the polymer film 200 b of FIG. 8A is disposed at 60 degrees angle of incidence relative to the light source 650. As can be seen, distortion is visible in the form of faint horizontal lines or bands in the machine direction (MD) of the polymer film 200 b, constituting dark and light regions that alternate in the transverse direction (TD).

FIG. 8B shows an example graphical representation of grayscale data of the image of FIG. 8A. The grayscale data may be taken from a cross-section of the shadow image corresponding to a slice of the polymer film 200 b in the transverse direction (TD) of the polymer film 200 b, namely the grayscale values of the 2500 pixels at the position of the vertical line 680 b in FIG. 8A. The grayscale values are shown on the y-axis of FIG. 8B for each pixel along the x-axis, with the resulting data presented as a line graph.

FIG. 8C shows an example graphical representation of a frequency spectrum of the grayscale data of FIG. 8B, which may be calculated by the computer 620 using an FFT algorithm, for example. As above, the frequency spectrum may represent the amplitude of the grayscale data at each spatial frequency, shown as cycles (peak-to-peak) per millimeter along the x-axis. As compared to the bare glass, the frequency spectrum calculated from the polymer film 200 b exhibits larger spikes, corresponding to the visible distortion in FIG. 8A. In particular, as can be seen in FIG. 8C, there are a few spikes having an amplitude of around 1, both above and below 2 cycles/mm.

FIG. 9A shows another example image of a shadow 670 c cast by a polymer film 200 c on the surface 660, with the polymer film 200 c being another example of the polymer film 200 shown in FIG. 6. FIGS. 9B and 9C show example graphical representations of grayscale data of the image of FIG. 9A and a frequency spectrum of the grayscale data, respectively, with the grayscale data being taken at the position of the vertical line 680 c in FIG. 9A. In the example of FIGS. 9A-9C, the polymer film 200 c is a 3-ply BOPET film sample. The polymer film 200 c is oriented so that its machine direction (MD) is horizontal and is disposed at 0 degrees angle of incidence (i.e. normal) relative to the light source 650. As can be seen, very little if any distortion is visible in FIG. 9A, with the frequency spectrum of FIG. 9C staying under 0.75 for all frequencies.

FIG. 10A shows another example image of a shadow 670 d cast by the same polymer film 200 c on the surface 660, with FIGS. 10B and 10C showing example graphical representations of grayscale data of the image of FIG. 10A and a frequency spectrum of the grayscale data, respectively. The grayscale data is taken at the position of the vertical line 680 d in FIG. 10A. FIGS. 10A-10C differ from FIGS. 9A-9C in that the polymer film 200 c is now disposed at 60 degrees angle of incidence relative to the light source 650, which is typical of usage on a windshield. Distortion in the form of machine direction (MD) bands is now plainly visible in the shadow 670 d of FIG. 10A. Correspondingly, the frequency spectrum of FIG. 10C exhibits several spikes, including a spike of 1.65 at 1.5 cycles/mm, a spike of 1.75 at 2.25 cycles/mm, and a spike of 1.5 at 4.1 cycles/mm. When the polymer film 200 c is mounted on a 60-degree car windshield, it is found that the degree of distortion is unacceptable.

FIG. 11A shows another example image of a shadow 670 e cast by a polymer film 200 e on the surface 660, with the polymer film 200 e being another example of the polymer film 200 shown in FIG. 6. FIGS. 11B and 11C show example graphical representations of grayscale data of the image of FIG. 11A and a frequency spectrum of the grayscale data, respectively, with the grayscale data being taken at the position of the vertical line 680 e in FIG. 9A. In the example of FIGS. 11A-11C, the polymer film 200 e is another 3-ply BOPET film sample. The polymer film 200 e is oriented so that its machine direction (MD) is horizontal and is disposed at 0 degrees angle of incidence (i.e. normal) relative to the light source 650. In the case of this polymer film 200 e, unlike the polymer film 200 c, there is already some visible banding indicating distortion at normal incidence. The frequency spectrum of FIG. 11C exhibits spikes above 1.25 at around 1 cycle/mm, accordingly. This relatively high degree of distortion is not so typical for products at normal incidence.

FIG. 12A shows another example image of a shadow 670 f cast by the same polymer film 200 e on the surface 660, with FIG. 12B showing an example graphical representation of grayscale data of the image of FIG. 12A. The grayscale data is taken at the position of the vertical line 680 f ₁ in FIG. 12A. FIGS. 12A and 12B differ from FIGS. 11A and 11B in that the polymer film 200 e is now disposed at 60 degrees angle of incidence relative to the light source 650, which is typical of usage on a windshield. The distortion that was already visible in the shadow 670 e at normal incidence is now much more severe, with the image in FIG. 12A being significantly banded in the machine direction (MD) and looking mottled. To better illustrate the severity of the distortion, FIG. 12C shows another example graphical representation of the grayscale data of the image of FIG. 12A, now in the form of an area plot of the dark and light bands in three dimensions. The grayscale data is taken at the position of the rectangle 680 f ₂ in FIG. 12D, which otherwise shows the same example image of FIG. 12A.

FIGS. 12E, 12F, and 12G show frequency spectra of the grayscale data of FIG. 12C (or 12A), taken along three different cross-sections at different locations along the machine direction (MD) about 10 mm apart. As can be seen, the spikes in the frequency spectra have an amplitude as high as 2 or are even in the 2.25-3.0 range in the case of FIG. 12F. It is also noted that the spikes are in the 1-2 cycles/mm range, which is thought to contribute to the high degree of distortion, as opposed to having spikes in the 2.5-3.5 cycles/mm range, for example. The polymer film 200 e has unacceptable distortion when mounted on a 60-degree car windshield.

FIG. 13A shows another example image of a shadow 670 g cast by a polymer film 200 g on the surface 660, with the polymer film 200 g being another example of the polymer film 200 shown in FIG. 6. FIGS. 13B and 13C show example graphical representations of grayscale data of the image of FIG. 13A and a frequency spectrum of the grayscale data, respectively, with the grayscale data being taken at the position of the vertical line 680 g in FIG. 13A. In the example of FIGS. 13A-13C, the polymer film 200 g is another 3-ply BOPET film sample. The polymer film 200 g is oriented so that its machine direction (MD) is horizontal and is disposed at 60 degrees angle of incidence relative to the light source 650. Distortion is visible in FIG. 13A in the form of horizontal bands, with associated spikes in the frequency spectrum of FIG. 13C. However, although the spikes reach an amplitude as high as 2, the large spikes are in the 2-4 cycles/mm frequency range, with the spikes below 2 cycles/mm having smaller amplitude of around 1. The polymer film 200 g is found to have acceptable distortion on windshield applications at 60-degree angle of incidence, and it is believed that this is due at least in part to the lack of significant spikes in the frequency domain below 2 cycles/mm.

In general, at 60-degrees angle of incidence, it has been found that a polymer film 200 should exhibit spikes of at most 1.0-1.2 in the frequency range below 2 cycles/mm. This low frequency range is found to correspond to a particularly critical pitch related to the optical distortion of the polymer film 200. Thus, preferably, the maximum of the frequency spectrum (e.g. the largest spike) is outside of the frequency range of 0.5-2 cycles/mm. To keep distortion minimal, it has been further found that from 2-4 cycles/mm, any spikes should be below 1.5 and from 4-6 cycles/mm any spikes can be as high as 2.0-2.5. This observed distortion in polymer films 200 at around 60 degrees and corresponding low-frequency spikes in the grayscale data of the above-described shadow images, which are not significant in bare glass, are believed to be due to the occurrence of random small changes in refractive index on the order of 0.010 caused by density variations across the polymer film as it is extruded and cooled during manufacturing. By feeding frequency spectrum data back into the manufacturing process, it is thus possible to tune the relevant process parameters to produce a polymer film having improved distortion characteristics at off-normal incidence. It is observed that the application of a hard coat does not appear to be the biggest contributor to distortion, whereas the use of an adhesive such as pressure sensitive adhesive (PSA) is a dominant contributor to distortion at high angle of incidence (e.g. 50-65 degrees) while not at low angle of incidence (e.g. 0-45 degrees).

Referring back to FIG. 6, the example extruder assembly 130 of the apparatus 600 may be the same as that of the apparatus 100 and may include a hopper 131 for loading polymer resin (e.g. pellets, beads, etc. which may include various materials to be compounded), a heater 133 for providing heat to the extruder assembly 130 to melt the resin, an extrusion screw 135 for moving the resin forward through one or more heated regions of the extruder assembly 130 (e.g. by rotating within a barrel), and a die 137 having a desired shape through which the melted resin is forced to produce the resulting polymer film 200. The polymer film 200 may thereafter be cooled and/or further shaped (including laminated/stacked to create a multi-ply polymer film 200) by one or more downstream rollers 140, eventually bringing the polymer film 200 to its final thickness and shape.

As noted above, it is believed that density variations across the polymer film as it is extruded and cooled cause changes in the index of refraction that result in the increased distortion found at higher angles of incidence. Therefore, it is contemplated that the apparatus 600, like the apparatus 100, may be configured to adjust one or more process parameters that affect the density variation of the polymer film 200 and/or the refractive index variation in the polymer film 200. As described above, relevant process parameters may include, for example, a temperature setting of the heater 133 used in melting the resin (e.g. absolute temperature or relative temperatures of a gradient or profile of a plurality of heated regions of the extruder assembly 130), a rotation speed of the extrusion screw 135 (which may determine melting time as well as degree of mixing of the resin), and/or a rotation speed of the one or more rollers 140 (which may determine cooling time and/or a degree of force acting on the polymer film 200 during or prior to cooling to stretch or otherwise shape the polymer film 200 in longitudinal and/or transverse directions while the polymer film 200 is still pliable). The relevant process parameters may additionally include parameters defining the application of pressure sensitive adhesive (PSA) or other adhesive (e.g. a flow speed thereof) used to produce a stack of films such as in the case of a multi-ply polymer film 200. The computer 620 may be programmed to adjust one or more such process parameters or any other relevant process parameters of the melting, extruding, shaping, or cooling based on a calculated frequency spectrum associated with the manufactured polymer film 200. In this way, the distortion of the polymer film 200 may be optimized for the intended angle of incidence at which the polymer film 200 will be used.

In the example of the apparatus 600 shown in FIG. 6, a surface 660 such as a white screen is set up behind the polymer film 200, and a light source 650 such as a collimated projector is positioned to illuminate the surface 660 with the polymer film 200 therebetween. In a case where the polymer film 200 is a distance of 12-36 inches (e.g. 24 inches) from the surface 660 as described above, the light source may be, for example, 64-120 inches, preferably 84-96 inches (e.g. 93 inches) from the polymer film 200. The image sensor 610 (e.g. a 40-megapixel imaging radiometer) may be positioned to capture an image of the shadow cast by the polymer film 200 on the surface 660. If the image sensor 610 is set up off-axis as described above, it may be positioned to the side 3-12 inches (e.g. 6.5 inches) away from the axis defined by the light source 650 and polymer film 200, for example. An aperture may be provided between the polymer film 200 and the surface 660 in order to cast a well-defined border shadow around the shadow of the polymer film 200 to be analyzed.

In order to obtain frequency spectra for different angles of the polymer film 200, multiple images may be taken with the polymer film 200 rotated relative to the light source 650. For example, during the capturing of a first image of the shadow 670 cast by the polymer film 200, the polymer film 200 may be at an angle of incidence relative to the light source 650 of 55-65 degrees (e.g. 60 degrees) and, during capturing of an additional image of the shadow 670 cast by the polymer film 200, the polymer film 200 may be at an angle of incidence relative to the light source 650 of less than 10 degrees (e.g. 0 degrees normal incidence). The computer 620 may then adjust the manufacturing process parameter(s) based on either or both of the frequency spectrum calculated from the first image and an additional frequency spectrum calculated from the additional image. It is contemplated that a sufficiently distortion-free film for use at off-normal incidence (e.g. for vehicle windshields) may exhibit certain properties in its frequency spectrum calculated from a 60-degrees angle of incidence shadow image. In particular, the frequency spectrum of a sufficiently distortion-free film may have its maximum value outside of 0.5-2 cycles/mm (e.g. above 2 cycles/mm), which is found to be a critical frequency range as described above. The preferred frequency spectrum of a usable film may, instead or additionally, exhibit specified maximum values for each of one or more predefined frequency ranges as further described above.

It is contemplated that the computer 620 may be programmed to adjust the process parameter(s) automatically without user input or in response to commands entered into a user interface of the computer 620. In this regard, the apparatus 600 may be set up to allow the image sensor 610 to capture images of the shadow 670 cast by the polymer film 200 in a continuous process. For example, the various images described above may be captured during or after cooling while the polymer film 200 is on the roller(s) 140. In the case of multiple images at different angles of incidence, multiple image sensors 610, light sources 650, and/or surfaces 660 may be set up at different stages or a single image sensor 610, light source 650, and/or surface 660 may automatically move to multiple positions. As the computer 620 calculates frequency spectra from the grayscale data of the captured images, the computer 620 may continuously adjust the relevant process parameters in order to keep the desired frequency spectra below specified values within specified spatial frequency ranges. Alternatively, the apparatus 600 may be set up to capture images and make adjustments to process parameters in a batch to batch process, either automatically or by manual operation. For example, after a polymer film 200 batch is completed (or during cooling), the relevant frequency spectra may be calculated and the computer 620 may make adjustments to the process parameters to improve the distortion characteristics of the next batch or to optimize the distortion characteristics of the next batch for a different purpose (e.g. to minimize distortion at a different range of angles of incidence).

FIG. 14 shows another example operational flow according to an embodiment of the present disclosure. Referring by way of example to the apparatus 600 shown in FIG. 6, the operational flow may begin with a step 1410 of melting a resin, a step 1420 of extruding the melted resin through a die to produce a polymer film 200, and steps 1430 and 1440 of shaping (e.g. stretching, applying adhesive, laminating, stacking) and cooling the polymer film 200. For example, steps 1410, 1420, 1430, and 1440 may be the same as steps 410, 420, 430, 440 of FIG. 4 and may be performed by an extruder assembly 130 and downstream roller(s) 140 as described in relation to FIGS. 1 and 6. During or subsequent to these steps, the operational flow may include a step 1450 of illuminating a surface 660 with a light source 650 and a step 1460 of capturing one or more images of a shadow 670 cast by the polymer film 200 on the surface 660 (e.g. using an image sensor 610). The images may be captured at various angles of incidence of the polymer film 200 relative to the light source 650 as desired, such as at 60 degrees, which is typical of usage of the polymer film 200 on a car windshield, and at 0 degrees normal incidence, for example.

Following the capturing of one or more images of the shadow 670 during or after steps 1410, 1420, 1430, and 1440, the operational flow may continue with a step 1470 of calculating one or more frequency spectra from the captured image(s) and a step 1480 of adjusting process parameter(s) based on the calculated frequency spectrum or spectra. In this way, the results of calculating the one or more frequency spectra associated with the polymer film 200 may be fed back into the manufacturing process to tune the optical distortion properties of the polymer film 200 being produced. The calculation of frequency spectra and/or adjusting of process parameter(s) may be performed by an appropriately programmed computer 620 as described above. In this regard, the computer 620, like the computer 120, may include a processor or programmable circuit (e.g. FPGA, PLA, etc.) for executing program instructions (e.g. software instructions, state information, etc.). The calculating step 1470 and/or adjusting step 1480 may be embodied in such program instructions and stored on a non-transitory program storage medium to be executed by the computer 620.

As described above, methods of adjusting manufacturing process parameter(s) based on both an MTF calculation and a frequency spectrum calculation are contemplated. In this regard, it should be noted that the polymer film 200 and its underlying materials and manufacturing processes (e.g. steps 410, 420, 430, 440, 1410, 1420, 1430, 1440), as well as its composition and purpose as a finished product, may be the same for both methodologies. The two methodologies may be used together in some cases, with the process parameter(s) being adjusted (manually or automatically) on the basis of both calculations. Whether they are to be used together or in the alternative, the two methodologies may be used to adjust the same process parameter(s) or respectively different process parameter(s).

The above description is given by way of example, and not limitation. Given the above disclosure, one skilled in the art could devise variations that are within the scope and spirit of the invention disclosed herein. Further, the various features of the embodiments disclosed herein can be used alone, or in varying combinations with each other and are not intended to be limited to the specific combination described herein. Thus, the scope of the claims is not to be limited by the illustrated embodiments. 

What is claimed is:
 1. A method of manufacturing a polymer film, the method comprising: melting a resin; extruding the melted resin through a die to produce a polymer film; shaping the polymer film; cooling the polymer film; illuminating a surface with a light source; capturing an image of a shadow cast by the polymer film on the surface while the polymer film is between the light source and the surface; calculating a frequency spectrum of at least a portion of the captured image; and adjusting a process parameter of the melting, the extruding, the shaping, or the cooling based on the frequency spectrum.
 2. The method of claim 1, wherein the process parameter is a temperature setting of a heater used in the melting.
 3. The method of claim 1, wherein the process parameter is a rotation speed of an extrusion screw used in the extruding.
 4. The method of claim 1, wherein the process parameter is a rotation speed of a roller used in the shaping or the cooling.
 5. The method of claim 1, wherein the polymer film is a multi-layer film, and the process parameter is a parameter for applying an adhesive used to stack two or more layers of the multi-layer film.
 6. The method of claim 1, wherein said calculating includes calculating the frequency spectrum of a cross-section of the shadow image corresponding to a slice of the polymer film in a transverse direction of the polymer film.
 7. The method of claim 1, wherein, during the capturing of the image, the polymer film is at an angle of incidence greater than 10 degrees with respect to the light source.
 8. The method of claim 7, wherein, during the capturing of the image, the polymer film is at an angle of incidence of 50-70 degrees with respect to the light source.
 9. The method of claim 8, wherein, during the capturing of the image, the polymer film is at an angle of incidence of 55-65 degrees with respect to the light source.
 10. The method of claim 9, further comprising: capturing an additional image of a shadow cast by the polymer film on the surface while the polymer film is between the light source and the surface at an angle of incidence less than 10 degrees with respect to the light source; and calculating an additional frequency spectrum of at least a portion of the captured additional image; wherein the adjusting is further based on the additional frequency spectrum.
 11. The method of claim 10, wherein, during the capturing of the additional image, the polymer film is at an angle of incidence less than 5 degrees with respect to the light source.
 12. The method of claim 1, wherein said adjusting is performed such that the frequency spectrum is kept below a first threshold within a first predefined frequency range.
 13. The method of claim 12, wherein said adjusting is performed such that the frequency spectrum is further kept below a second threshold within a second predefined frequency range.
 14. The method of claim 1, wherein said adjusting is performed such that a maximum of the frequency spectrum is kept outside of a predefined frequency range.
 15. The method of claim 14, wherein the predefined frequency range is 0.5-2 cycles per millimeter.
 16. The method of claim 1, wherein the polymer film is a biaxially-oriented polyethylene terephthalate film.
 17. The method of claim 1, wherein the polymer film is a thermoplastic polyurethane film.
 18. The method of claim 1, wherein the polymer film is a biaxially-oriented polypropylene film.
 19. The method of claim 1, wherein the polymer film has a visible light transmission (VLT) greater than 89%.
 20. The method of claim 1, wherein the process parameter affects a density variation of the polymer film.
 21. The method of claim 1, wherein the process parameter affects a refractive index variation in the polymer film.
 22. The method of claim 21, wherein the process parameter affects a frequency of refractive index changes on the order of 0.010 in the polymer film.
 23. A polymer film having a density variation such that a maximum value of a frequency spectrum calculated from grayscale image data of a transverse cross-section of a shadow cast by the polymer film at 60 degrees angle of incidence with respect to a light source is at a spatial frequency greater than 2 cycles per millimeter.
 24. An apparatus for manufacturing a polymer film, the apparatus comprising: an extruder assembly for melting a resin and extruding the melted resin through a die to produce a polymer film; a roller for shaping and/or cooling the polymer film; a light source for illuminating a surface; an image sensor for capturing an image of a shadow cast by the polymer film on the surface while the polymer film is between the light source and the surface; and a computer for calculating a frequency spectrum of at least a portion of the captured image and adjusting a process parameter of the extruder assembly or the roller based on the calculated frequency spectrum. 